enphase microinverter grid sync method

How do the enphase microinverters detect the gridsignal so that they can stop generation during an outage? My micros are connected to my panel with 2x20a breakers and I imagine they could be sensing there. But there is also an enphasegateway device that is connected to the internet. This device is plugged into a standard outlet and it comminicates to the microinverters using the powerline which is a bidirectional dataprotocol over an AC line. I was thinking that it's also possible that the gateway sends out a heartbeat using the powerline protocol to the panels and if the panels don't see that heartbeat, then they stop generating. Then if the grid when down, the gateway loses power and the heartbeat stops. Anyone know?

Hi
The anti islanding is done at the microinverter level, they can happily operate without a functioning gateway.